1. Field of the Invention
The present invention relates generally to batch digestors and more particularly to apparatus for recovering heat from batch digester cooking fluids and a process for use thereof.
2. Description of the Prior Art
Batch digesters are commonly used in industrial applications to liberate fibrous pulp or other desired by-products from woodchips or other fibrous materials by digestion or cooking. The digestion of materials in batch digesters requires the use of heat and a highly corrosive fluid or cooking "liquor", such as sodium sulphite. To effect digestion a quantity of material is placed within a digester. This material may be preheated, exposed to a predetermined quantity of super heated cooking liquor, allowed to soak or "cook" in the cooking liquor at an elevated temperature and pressure for a predetermined time period, and then discharged from the digester and separated into the desired product and spent liquor. The spent cooking liquor is the contaminated cooking liquor which includes residue pulp liberated during the cooking process. Release of the pressure in the digester liberates energy contained in the superheated contents of the digester. Such energy may be subsequently recovered and utilized if desired.
The reclamation of heat from batch digester systems is well known in the prior art. Specifically U.S. Pat. Nos. 1,832,367 issued to T. L. Dunbar, Nov. 17, 1931; 1,860,755 issued to Stevens et al, May 31, 1932; 1,885,561 issued to Stevens et al, Nov. 1, 1932; 1,947,889 issued to C. B. Thorme, Feb. 20, 1934; and 2,216,649 issued to A. D. Merrill, Oct. 1, 1940 all discuss apparatus and processes for such heat recovery. The prior art teaches the use of direct or indirect heat exchange to transfer heat between the cooking and spent liquors. Such a heat exchange is to preheat the cooking liquor prior to its introduction into the digester. Preheating the cooking liquor in such a manner reduces the expense of independently heating the cooking liquor and reduces total processing times.
Direct heat exchange in the prior art between cooking and spent liquors is accomplished by mixing heated spent liquor with unheated cooking liquor. A major drawback to direct heat exchange is the contamination of the cooking liquor by the spent liquor resulting in lengthened cooking times.
The indirect heat exchange between the cooking and spent liquors is the preferred approach of the prior art. The indirect heat exchange between the cooking and spent liquors is accomplished by use of a fluid-to-fluid heat exchanger. Unheated cooking liquor is introduced into one portion of the heat exchanger while heated spent liquor is introduced into a second portion of the heat exchanger. The two liquors are passed near each other inside their respective portions of the heat exchanger. The heat exchange is accomplished through the walls of the heat exchanger. Neither cooking or spent liquor is allowed to contact the other liquor and contamination of the cooking liquor is avoided. Once heated, the cooking liquor is superheated. When superheated, cooking liquor is stored for future use in a pressurized accumulator to prevent release of energy from the superheated liquor.
Conventional fluid-to-fluid heat exchangers have an acccessible fluid system and an inaccessible fluid system. Both systems may be washed or flushed, but only the accessible system may be truly cleaned or descaled in case of blockage. If blockage occurs due to heavy scaling or fouling in the inaccessible portion of the heat exchanger, the heat exchanger must be replaced or rebuilt. Such replacement or rebuilding of a heat exchanger is expensive.
A disadvantage of the prior art is that it requires highly corrosive liquors to pass through the inaccessible portion of the heat exchanger resulting in fouling, scaling and general blockage of that portion of the heat exchanger. Such blocking, scaling or fouling requires the expensive rebuilding, replacement or cleaning of the heat exchanger to restore efficient heat exchanger operation.
Another disadvantage of the prior art is that it requires the use of expensive pressurized accumulators to store heated liquors.
A further disadvantage of the prior art is that it requires the use of expensive pressurized measuring equipment to accurately measure the quantity of preheated cooking liquor introduced into the digester.